Arsenical copper ores or concentrates typically include arsenic in the form of enargite, Cu3AsS4, tennantite, Cu6[Cu4(Fe, Zn)2]As4S13, and/or collusite Cu12VAs3S16. These ores or concentrates also typically include other sulfide minerals, such as pyrite, and lesser amounts of chalcopyrite, chalcocite, sphalerite and/or galena. Such ores may also include economically significant quantities of gold and silver.
Arsenical copper concentrates may be processed by pyrometallurgical processes, however such processes suffer from high costs and environmental challenges, requiring complex gas handling systems and treatment processes before the arsenic can be disposed of in an environmentally acceptable manner. A few operations exist where higher arsenic-containing concentrates are first subjected to a controlled oxidizing roast to remove a significant portion of the arsenic as As2O3-containing dusts along with some of the sulfur as SO2, prior to more conventional processing of the reduced arsenic concentrates. The collected As2O3 dust is problematic, however, due to a lack of market uses. In some instances, such As2O3 dusts are dissolved in water and then precipitated as a ferric arsenate, with addition of iron sulfates and oxidation, and then impounded. As a consequence, copper smelting operations are either not able to process arsenical copper concentrates, or transfer the high additional processing costs to the concentrate producers, hence restricting the production of concentrates from arsenical copper ores, and the recovery of copper therefrom.
Enargite and tennantite are refractory copper minerals that are generally considered difficult to treat hydrometallurgically. Hydrometallurgical oxidative processes have been studied, developed, and proposed for arsenical copper concentrates for several decades. However, there are no commercial operations to date, in spite of various successes claimed at the research level. Hydrometallurgical oxidation of arsenical copper concentrates, including enargite and/or tennantite under acidic atmospheric conditions, is characterized by slow leaching kinetics, largely due to the formation of elemental sulfur that creates a passivation layer that poses a barrier to copper dissolution. As a result, required leaching time to recover copper is extremely long. In addition, the arsenic must be disposed of in an environmentally stable form.
Attempts have been made to leach enargite-containing copper concentrates by acid oxidative leaching under atmospheric conditions or in autoclaves at low, medium, or high temperatures, as well as by bioleaching or alkaline leaching methods. With respect to alkaline leaching, under non-oxidative conditions with NaOH or NaOH/Na2S solutions, the objective is a selective leach of the arsenic, leaving behind a low-arsenic copper sulfide product suitable for treatment by conventional pyrometallurgy. The major drawback and problem with this “selective” leach is dealing with the resultant arsenic-containing liquor. In attempts to commercially utilize this approach, arsenic was precipitated as copper arsenate by addition of copper sulfate. Other proposed processes suggested arsenic precipitation as calcium arsenate, which is environmentally much less desirable than scorodite.
Scorodite, which is a crystalline form of ferric arsenate, FeAsO4.2H2O, is generally recognized as a stable compound for arsenic disposal. Iron must be present in the Fe-III oxidation state and arsenic in the As-V oxidation state to enable the precipitation of ferric arsenate compounds. Scorodite formation can be induced even under elevated sulfuric acid conditions, requiring a minimum molar Fe-III/As-V ratio of only 1:1, the presence of scorodite seed and elevated temperatures. In contrast, low acidity conditions (pH greater than 2) are required when amorphous ferric arsenate co-precipitates are produced, requiring molar Fe-III/As-V ratios of greater than 2:1 and preferably greater than 3:1 to produce precipitates of acceptable environmental stability.
In atmospheric oxidative leaching of arsenical copper concentrates in acidic ferric sulfate solutions, the arsenic may be removed by precipitation either during leaching or from the oxidative leach solution, i.e., after leaching and solid-liquid separation. Methods for arsenic precipitation as ferric arsenate generally suffer from drawbacks, however, such as losses of copper, excessive loss of soluble iron, loss of acid, and the use of excess neutralizing reagents.
Thus, hydrometallurgical processes for the extraction of copper from arsenical copper sulfide concentrates are generally not considered practical or economical.
Improvements in the extraction of copper from arsenical copper sulfide concentrates are desirable.